VISIBLE LIGHT DETECTOR WITH HIGH-PHOTORESPONSE BASED ON TiO2/MoS2 HETEROJUNCTION AND PREPARATION THEREOF

ABSTRACT

In the field of photoelectric devices, a visible light detector is provided with high-photoresponse based on a TiO2/MoS2 heterojunction and a preparation method thereof. The detector, based on a back-gated field-effect transistor based on MoS2, includes a MoS2 channel, a TiO2 modification layer, a SiO2 dielectric layer, Au source/drain electrodes and a Si gate electrode, The TiO2 modification layer is modified on the surface of the MoS2 channel. By employing micromechanical exfoliation and site-specific transfer of electrodes, the method is intended to prepare a detector by constructing a back-gated few-layer field-effect transistor based on MoS2, depositing Ti on the channel surface, and natural oxidation.

CROSS REFERENCE TO RELATED APPLICATION

This application claims foreign priority benefits under 35 U.S.C. §119(a)-(d) to CN Application 202010631397.X filed Jul. 3, 2020, which ishereby incorporated by reference in its entirety.

TECHNICAL FIELD

The present disclosure relates to the field of photoelectric devices,and specifically relates to a visible light detector withhigh-photoresponse based on TiO₂/MoS₂ heterojunction and a preparationmethod thereof.

BACKGROUND

With the development of photoelectric devices towards highly horizontalintegration and continuous longitudinal thinning, the commercialapplication of silicon-based field-effect transistors is facing thebottleneck of further miniaturization. The emergence of two-dimensionaltransition metal dichalcogenides (TMDCs) inspires new hope for thecontinuation of Moore's Law. TMDCs have shown very important applicationprospects in microelectronic and optoelectronic fields includingfield-effect transistors, light emitting diodes, photodetectors, and thelike because of their excellent physical characteristics such as tunableenergy band structure, strong excitonic emission, high on/off ratio ofcurrent, etc. However, the low photo-responsivity and low specificdetectivity of the photodetector based on a single-component of TMDCsare difficult to meet the needs in practical applications. Therefore, itis an important research field at present to design and fabricatehigh-performance photodetectors based on TMDCs heterostructures atnanoscale. In particular, there are still many opportunities for thedevelopment of metal oxides/TMDs photodetectors with high photoresponse.

In the heterostructures of metal oxides/TMDCs, different interfacecharge effects, including charge transfer, charge traps, dielectricscreening, piezoelectric effects and the like, can be used to optimizethe photoelectric properties and promote their application potentials inthe optoelectronic field. The interface charge effects between metaloxides and TMDCs are closely related to the interface states. Metaloxides/TMDCs heterojunction has controllable surfaces and interfaces, aswell as unique energy band alignment, so it is expected to become one ofthe most promising photoelectric functional materials. The significantlyenhanced photoresponse performance of the photodetector based on metaloxides/TMDCs heterojunction not only provides a good foundation for itsapplication in photoelectric detectors, but also inspires a new idea forconstructing novel high performance photodetectors based on TMDCs.

SUMMARY

To overcome the deficiencies of the prior art, the present disclosure isintended to provide a visible light detector with high-photoresponsebased on TiO₂/MoS₂ heterojunction and a preparation method thereof.

The present disclosure employs the following technical schemes:

A visible light detector with high-photoresponse based on TiO₂/MoS₂heterojunction, the detector is based on a back-gated field-effecttransistor based on MoS₂, the detector includes a MoS₂ channel, a TiO₂modification layer, a SiO₂ dielectric layer, Au source/drain electrodesand a Si gate electrode, the TiO₂ modification layer is modified on thesurface of the MoS₂ channel. In the present disclosure, we employ amicromechanical exfoliation method to prepare MoS₂ flakes. Through thismethod, layer-structured bulks are exfoliated by using adhesive tapes,which would not destroy the covalent bonds in the plane of MoS₂, and theresulting MoS₂ generally has the characteristics of few defects, highcrystallinity and clean surface. However, the preparation of MoS₂ flakesis not restricted to the micromechanical exfoliation method.Single-layer or few-layer MoS₂ prepared by a chemical vapor depositionmethod can also be used instead. To obtain smooth and uniform metaloxide nanostructures, a certain thickness of Ti is deposited on thesurface of MoS₂ by using e-beam evaporation, and then oxidized naturallyto get TiO₂. Because the e-beam evaporation has the advantages of highenergy density and controllable depositing rate compared with otherphysical vapor deposition methods, so it can be used to preparethin-film materials with high purity and high uniformity.

Furthermore, the MoS₂ channel is few-layer MoS₂ flakes with a highcrystallinity.

Furthermore, the MoS₂ flakes are in a hexagonal phase with asingle-crystal structure, showing semiconducting properties;

Furthermore, the few-layer means 3 layers, and the overall thickness is2-2.5 nm.

Furthermore, the lateral dimension of the MoS₂ flakes is at least 10 μm.

Furthermore, the TiO₂ modification layer is a naturally oxidized TiO₂layer.

Furthermore, the thickness of the TiO₂ layer is 1-2 nm.

Furthermore, the TiO₂ layer is in a crystalline state or an amorphousstate, and when the TiO₂ layer is in the crystalline state, it hassingle-crystal sheets, and the sheet dimension is 2-3 nm.

Furthermore, at a zero-gate voltage and under the illumination of awhite-light LED, the detector can reach a high photoresponsivity of 1099A/W and a high specific detectivity of 1.67×10¹³ Jones.

The present disclosure also provides a method of preparing the visiblelight detector with high-photoresponse based on TiO₂/MoS₂heterojunction, including the following steps:

S1. preparing MoS₂ flakes, and transferring the MoS₂ flakes onto aSiO₂/Si wafer;

S2. construction of a transistor based on MoS₂: site-specifictransferring gold electrodes onto the MoS₂ flakes obtained in step S1,getting source/drain electrodes of the detector; the highly-doped Sisubstrate is a gate electrode;

S3. e-beam evaporation of Ti: depositing a certain thickness of metallicTi film on the channel surface of the transistor based on MoS₂constructed in step S2, getting a device based on Ti/MoS₂heterojunction;

S4. natural oxidation: exposing the device based on Ti/MoS₂heterojunction prepared in step S3 in air for oxidation, obtaining aTiO₂/MoS₂ heterojunction for a visible-light detector.

Furthermore, in step S1, the MoS₂ flakes are prepared by amicromechanical exfoliation method, and the MoS₂ flakes are heated afterbeing transferred onto the SiO₂/Si wafer.

Furthermore, the micromechanical exfoliation method is to tear and stickbulk MoS₂ crystals repeatedly by using adhesive tapes, getting adhesivetapes attached with MoS₂ thin layers, which are transferred onto theSiO₂/Si wafer.

Furthermore, the SiO₂/Si wafer attached with MoS₂ on adhesive tapes isheated on a heating plate at 100° C. for 2 min.

Furthermore, in step S2, the electrode thickness of the transistor basedon MoS₂ is 50 nm, and after the transistor based on MoS₂ is constructed,it is annealed at 200° C. in an atmosphere of Ar/H₂ at 10 Pa for 1 h.

Furthermore, in step S3, the thickness of the e-beam-evaporated Ti filmis 2 nm, and the depositing rate is 0.2 Å/s.

The present disclosure provides a controllable method of constructing aphotodetector with high-photoresponse based on TiO₂/MoS₂ heterojunction,in which a micromechanical exfoliation method is firstly employed to getMoS₂ flakes, then a back-gated field-effect transistor based on MoS₂ isconstructed by a process of transferring the electrodes, followed bydepositing metal Ti on the channel surface, and finally naturaloxidation to get the photodetector based on TiO₂/MoS₂.

The photodetector based on TiO₂/MoS₂ prepared in the present disclosurehas both high photoresponsivity and high specific detectivity,significantly improving the MoS₂ visible light detection performance,and effectively promoting the further development of photodetectorsbased on transition metal dichalcogenides.

The present disclosure has the following innovations and beneficialeffects:

1. Compared with the photodetector based on 73-layer MoS₂ prepared by apulse laser deposition method, the few-layer MoS₂ exfoliatedmechanically in the photodetector based on TiO₂/MoS₂ has an innovationin the channel thickness, in which the channel thickness of thetransistor is reduced by 96%.

2. Compared with the traditional way of evaporating electrodes, theconstruction of a transistor by site-specific transferring electrodescan obtain a relatively perfect contact interface, so that the devicecan display good Ohmic output characteristics, which is conducive to thetransport of photo-generated carriers between the electrodes and MoS₂.

3. The acquisition of TiO₂ modification layer by the natural oxidationof Ti is a major innovation of the present disclosure. This method canavoid the damage to the MoS₂ lattice or the introduction of chemicalimpurities, compared with the direct evaporation of TiO₂ or spin coatingof TiO₂ synthesized by chemical methods; moreover, due to the strongwettability of Ti on the surface of MoS₂, ultrathin and layered TiO₂ canbe obtained, thus increasing the contact area between TiO₂ and MoS₂. Inaddition, the incomplete oxidation of Ti allows more oxygen vacancies tobe generated in TiO₂, thus producing significant responses to visiblelight.

4. The study on the photo-responsive behavior of the photodetector basedon TiO₂/MoS₂ to white light is also a major innovation of the presentdisclosure. The traditional photodetector based on MoS₂ generallyexplores the photoresponse to the light with a single wavelength, whilethe present study provides a foundation for the application of thephotodetector based on MoS₂ in the field of visible light detection.

5. The materials used in the preparation method are cost-effective, theoperation procedure is simple and controllable, the products haveuniform morphology and sizes, thus overcoming the disadvantages ofcomplex processing procedure, tedious steps, and harsh conditionsrequired in the existing technologies, thereby greatly reducing the costcompared with the physical vapor deposition of TiO₂ and the electrodesmade by e-beam lithography.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing the preparation process of avisible light detector with high-photoresponse based on TiO₂/MoS₂heterojunction according to one example of the present disclosure.Where: (a) shows the few-layer MoS₂ flake prepared by micromechanicalexfoliation; (b) shows the back-gated field-effect transistor based onMoS₂ constructed by transferring the electrodes; (c) shows thetransistor based on Ti/MoS₂ generated by e-beam evaporation of Ti; (d)shows the photodetector based on TiO₂/MoS₂ heterojunction afteroxidation.

FIG. 2A shows an optical image of the few-layer MoS₂ flakes.

FIG. 2B shows an atomic force microscope image of the MoS₂heterostructure modified with TiO₂.

FIG. 3A shows a high-resolution transmission electron microscope imageof the TiO₂/MoS₂ heterojunction.

FIG. 3B shows a fast Fourier transform pattern of the high-resolutiontransmission electron microscope image containing the region as shown inFIG. 3 a.

FIG. 3C shows an inverse fast Fourier transform pattern of thediffraction ring from TiO₂ in FIG. 3 b.

FIG. 3D shows an electron energy loss spectroscopy of the TiO₂/MoS₂heterojunction, showing the signal from Ti.

FIG. 4 shows an X-ray photoelectron spectroscopy of the TiO₂/MoS₂heterojunction, where a-d show core-level high resolution XPS spectra ofMo 3d, S 2p, Ti 2p and O1s in TiO₂/MoS₂, respectively.

FIG. 5A shows an optical image of the field-effect transistor based onMoS₂.

FIG. 5B shows a Raman spectrum of the MoS₂ flake.

FIG. 5C shows a 3D-model diagram of the photodetector based on MoS₂modified with TiO2 nanosheets.

FIG. 5D shows a 3D-model and cross-section diagram of the photodetectorbased on MoS₂ modified with TiO₂ nanosheets.

FIG. 6A shows transfer curves of photodetectors based on MoS₂ andTiO₂/MoS₂ respectively in dark and under different illumination powerdensities at a source/drain bias of 1 V, with the inset showing thedependence of threshold voltage variations on the power density.

FIG. 6B shows the photocurrents as a function of the gate voltages underdifferent power densities.

FIG. 6C shows the dependence of the photocurrents on the power densityat different gate voltages.

FIG. 7A shows the dependence of the responsivities of photodetectorsbased on MoS₂ and TiO₂/MoS₂ respectively on the power density atdifferent gate voltages.

FIG. 7B shows the dependence of the specific detectivities ofphotodetectors based on MoS₂ and TiO₂/MoS₂ respectively on the powerdensity at different gate voltages.

FIG. 7C shows the responsivity comparison of the two photodetectorsbased on MoS₂ and TiO₂/MoS₂ respectively at different gate voltages.

FIG. 7D shows the specific detectivity comparison of the twophotodetectors based on MoS₂ and TiO₂/MoS₂ respectively at differentgate voltages.

FIG. 8A shows an energy band diagram of a triple-layer MoS₂ undervisible light illumination.

FIG. 8B shows an energy band diagram of the TiO₂/MoS₂ heterojunctionunder visible light illumination.

DETAILED DESCRIPTION

Specific examples of the present disclosure will be described in detailin combination with the attaching drawings below. It should be notedthat the technical features or the combination thereof described in thefollowing examples should not be considered to be isolated, which can becombined to achieve better technical effects.

Example 1

This example provides a visible light detector with high-photoresponsebased on TiO₂/MoS₂ heterojunction, the detector is based on a back-gatedfield-effect transistor based on MoS₂, and the detector includes a MoS₂channel, a TiO₂ modification layer, a SiO₂ dielectric layer, Ausource/drain electrodes and a Si gate electrode, the TiO₂ modificationlayer is modified on the surface of the MoS₂ channel. At a zero-gatevoltage and under the illumination of a white-light LED, the detectorcan reach a high photoresponsivity of 1099 A/W and a high specificdetectivity of 1.67×10¹³ Jones.

Preferably, the MoS₂ channel is few-layer MoS₂ flakes with a highcrystallinity, the MoS₂ flakes are in a hexagonal phase with asingle-crystal structure; the few-layer means 3 layers, and the overallthickness is 2-2.5 nm.

Preferably, the TiO₂ modification layer is a naturally oxidized TiO₂layer, and the thickness of the TiO₂ layer is 1-2 nm.

Preferably, the TiO₂ layer is in a crystalline state or an amorphousstate, and when the TiO₂ layer is in the crystalline state, it hassingle-crystal sheets.

Example 2

This example provides a method of preparing the visible light detectorwith high-photoresponse based on TiO₂/MoS₂ heterojunction.

S1. Preparation of few-layer MoS₂ flakes with nanometer thickness andhigh crystallinity;

S1.1 Taking an appropriate amount of bulk MoS₂ crystals and placing themon one adhesive side of Scotch tapes;

S1.2 Folding both ends of the adhesive tapes in half along the middle,contacting the other adhesive side of the adhesive tapes with the uppersurface of MoS₂ bulks, compacting the part of adhesive tapes attachedwith MoS₂ gently, tearing slowly so that the MoS₂ bulks can be dividedinto two parts, repeating multiple times of sticking and tearing, untilsmall pieces of MoS₂ were distributed on both sides of the adhesivetapes discretely;

S1.3 Cutting the adhesive tapes in the middle, attaching the sideattached with MoS₂ slowly onto SiO₂/Si substrates that have been washedwith piranha solution, flattening and squeezing the adhesive tapes,heating the SiO₂/Si wafer attached with MoS₂ on adhesive tapes on aheating plate at 100° C. for 2 min to enhance the adhesion between MoS₂and the substrates;

S1.4 Tearing the adhesive tapes slowly from the silicon wafer, resultingin that the MoS₂ flakes being transferred onto the substrates (FIG.1(a)).

S2. Preparation of field-effect transistor based on MoS₂

S2.1 Cutting silicon wafers plated with an array of gold electrodes at athickness of 50 nm into rectangular chips of 0.5 cm×1 cm by using asilicon knife;

S2.2 Dropwise adding an appropriate amount of PMMA solution onto thesurface of chips with electrodes, spin-coating at a low rate of 1000r/min for 10 s firstly, then spin-coating at a high rate of 4000 r/minfor 50 s; placing the chips with electrodes spin-coated with PMMA on aheating plate and heating at 120° C. for 3 min for solidification,scraping PMMA off the edge of silicon wafers with a knife in case ofblocking the etching of the oxidation layer;

S2.3 Immersing the electrode chips with solidified PMMA right side up ina HF solution (the volume ratio of HF to H₂O is 1:3), etching at roomtemperature for 2 h, taking them out with acid and alkali resistanttweezers and rinsing with deionized water for many times; covering apiece of PDMS over the electrodes, and exfoliating the electrodes offthe silicon wafers slowly;

S2.4 With the use of a probe station, placing MoS₂ flakes in the spotregion of the microscope on a translation stage, adjusting the opticalmicroscope while adjusting the X-Y-Z translation stage to stack theelectrodes on MoS₂ flakes accurately, heating at 140° C. for 3 min tomake PDMS to be softened and lose viscosity, exfoliating PDMS off sothat the PMMA films attached with electrodes can be transferred onto theMoS₂ flakes; then heating the device at 180° C. for 3 min to enhance theadhesion between the electrodes and MoS₂;

S2.5 Immersing the device attached with PMMA films in an acetonesolution for 2 h to dissolve PMMA, and washing the device withisopropanol and deionized water successively;

S2.6 Placing the device in a tubular furnace, introducing a mixedatmosphere of Ar/H₂ at 40 sccm, maintaining the pressure within thefurnace at 10 Pa, annealing at 200° C. for 1 h so as to further removePMMA and enhance the contact between the gold electrodes and MoS₂, thusobtaining the field-effect transistor based on MoS₂ (FIG. 1(B)).

S3. Deposition of metal Ti

S3.1 Taking an appropriate amount of Ti target material particles into acrucible, and sticking the wafers with MoS₂ samples onto a substratewith high-temperature adhesive tapes.

S3.2 Setting the thickness of the deposited Ti film to be 2 nm and thedepositing rate to be 0.2 Å/s.

S3.3 When the base pressure reached 1×10⁻⁴ Pa, heating the targetmaterials with e-beam; when the depositing rate on the film thicknessgauge was steady, opening the middle baffle, so that Ti can be depositedon the surface of the device channel at room temperature (FIG. 1(c)).

S4. Natural oxidation of Ti

A Ti/MoS₂ heterojunction device will be oxidized to a TiO₂/MoS₂heterojunction device quickly when exposed in air (FIG. 1(D)), which wasthen stored in a vacuum vessel at 2000 Pa.

Example 3

1. Characterization of the Visible Light Detector withHigh-Photoresponse Based on TiO₂/MoS₂ Heterojunction

The surface morphology of the samples was characterized by using aBruker Multimode 8 atomic force microscope (AFM). The microstructure ofthe samples was characterized by using a JEOL 2200FS transmissionelectron microscope (TEM) equipped with electron energy lossspectroscopy (EELS). X-ray photoelectron spectroscopy (XPS) washarvested by using a PHI 5000 VersaProbe III X-ray photoelectronspectrometer. Raman spectrum (Raman) was harvested by using a HoribaJobin Yvon HR-800 micro-Raman system excited with 532 nm laser.

2. Photoresponse Test

The photoresponse of the photodetector based on TiO₂/MoS₂ heterojunctionwas tested in air by using a B1500A semiconductor parameter analyzer(Agilent). LED visible lights for test mainly include visible lights atthree wavelengths of 450 nm, 541 nm and 715 nm. The spot diameter was 3mm, far greater than the length and width of the channel of the device.

3. Results and Discussion

Firstly, few-layer MoS2 flakes were obtained by exfoliation (FIG. 2a ),and upon the completion of TiO₂ modification, the surface morphology ofTiO₂/MoS₂ heterojunction was characterized by AFM. As shown in FIG. 2B,the surface of MoS₂ modified with TiO₂ was smooth and clean, theroot-mean-square roughness was about 0.3 nm, and there were nosignificant island particles, indicating the layered growth of Ti onMoS₂.

The high resolution transmission electron microscope (HRTEM) image showsthe microstructure of TiO₂/MoS₂ heterojunction (FIG. 3A), indicatingthat MoS₂ has a good crystallinity. The FFT image containing the regionas shown in FIG. 3A is shown in FIG. 3B, showing the hexagonal symmetricdiffraction spots of MoS₂. In addition, a weak diffraction ringcorresponding to a plane with a lattice spacing of 0.218 nm can beobserved. FIG. 3C shows an IFFT image of the polycrystalline ring, fromwhich we can observe the two nanosheets (NSs) in the region as shown inFIG. 3A. The sheet on the upper left is roughly a square 2-3 nm on aside, and the spacing of crystal faces in two orthogonal directions is0.219 nm (OA) and 0.213 nm (OB) respectively. The amorphous layerbetween the nanosheets may be amorphous TiO₂. To further determine thecomponents of the nanosheets, an EELS analysis was conducted. As shownin FIG. 3D, significant Ti-L₃, Ti-L₂ and O-K peaks can be observed,which are located at 458.8 eV, 463.6 eV and 532.4 eV respectively,demonstrating the formation of TiO₂ on the surface of MoS₂.

The electronic state and interfacial interaction of TiO₂/MoS₂ werecharacterized by using XPS. In FIG. 4, (A) and (B) showed Mo 3d_(5/2),Mo 3d_(3/2), S 2p_(3/2), and S 2p_(1/2) doublet peaks, in which the peakpositions were consistent with those in XPS spectrum of pristine MoS₂.The spectrum of Ti 2p core level was shown in FIG. 4(C), the doubletpeaks of Ti 2p_(3/2) and Ti 2p_(1/2) were detected at 458.78 eV and464.50 eV, which corresponds to the binding energies of Ti⁴⁺ in TiO₂.The two satellite peaks at 457.20 eV and 463.40 eV may arise from Ti³⁺caused by the oxygen vacancies in TiO₂. This result further confirmedthat Ti on MoS₂ was oxidized to TiO₂, and there was no interfacialreaction between Ti and MoS₂. In the spectrum of 0 is core level asshown in FIG. 4(D), the peak at 531.50 eV derives from the oxygenvacancies in TiO₂.

The optical image of the field-effect transistor based on MoS₂ is asshown in FIG. 5A, in which the length and width of the channel are 7 μmand 17 μm respectively, and the area of the device is about 119 μm². Thefrequency difference between the two Raman characteristic peaks E_(2g) ¹and A_(1g) is 23.3 cm⁻¹ (FIG. 5B), indicating that the MoS₂ flake hastriple layers. After conducting a photoresponse test on thephotodetector based on MoS₂, Ti with a thickness of 2 nm was depositedon the channel, which is oxidized naturally to get the photodetectorbased on TiO₂/MoS₂ heterojunction, its 3D-model and cross-sectiondiagram were shown in FIG. 5C and FIG. 5D.

FIG. 6A shows transfer curves of two photodetectors in dark and underdifferent illumination power densities. With the increase of the powerdensity of white-light, the transfer curve of the photodetector based onMoS₂ moves towards the negative direction gradually, and the thresholdvoltage changes, indicating that the photogating effect dominates thephotoresponse. Compared with the pure MoS₂ device, the threshold voltagevariation of the detector based on TiO₂/MoS₂ is more significant, asshown in the inset of FIG. 6A. It indicates that TiO₂ enhances thephotogating effect of MoS₂, of which the enhancement mechanism will bediscussed later. FIG. 6B shows the photocurrents as a function of thegate voltages for the two photodetectors. At the same power density, thephotocurrent of the detector based on TiO₂/MoS₂ is significantly greaterthan that of the device based on MoS₂. Compared with the photodetectorbased on pure MoS₂, the photocurrent of the device based on TiO₂/MoS₂increases continually with the increase of the gate voltage, this is dueto that under light illumination, electrons in TiO₂ are injected intoMoS₂, increasing the electron concentration in MoS₂, thereby increasingthe channel current. The interface charge transfer behavior will bediscussed in the photoresponse mechanism later.

FIG. 6C describes the dependence of the photocurrents on the powerdensity at different gate voltages, in which the dependence of thephotocurrents on the power density is fitted by using an index-lawequation. For the photodetector based on pure MoS₂, under a reverse biasof −40 V, the photocurrent increases almost linearly with the increaseof the power density (a≈1.03), indicating that the photo-generatedcarriers are mainly determined by the flux of incident photon, and thephotoconductive effect plays a key role in the generation ofphotocurrent. However, under a forward bias of 60 V, it is observed thatthere is a strong sublinear relationship between the photocurrent andthe power density (α=0.35), indicating that the photogating effectdominates the generation of photocurrent. Under light illumination,photo-generated holes were then captured by traps and generate a localelectric field, thereby causing the transfer curve to shift towards thenegative direction. A similar dependence of the photocurrent on thepower density was also found in the photodetector based on TiO₂/MoS₂.

FIG. 7A describes the dependence of the responsivities of photodetectorsbased on MoS₂ and TiO₂/MoS₂ respectively on the power density atdifferent gate voltages. The responsivity of the photodetector based onTiO₂/MoS₂ is significantly higher than that of the device based on MoS₂.The responsivity decreases almost monotonically with the increase of thepower density, this is because the trapped charges in MoS₂ have becomesaturated, thus confirming the dominant mechanism of photogatingeffects. FIG. 7C compares the responsivities of the two photodetectorsat different gate voltages. At zero bias voltage, at a power density of23.2 μW/mm², the responsivity of the photodetector based on TiO₂/MoS₂reaches up to 1099 A/W, which is about 1.7 times that of a detectorbased on pure MoS₂ (406 A/W). The responsivity can be further enhancedby applying a higher bias voltage.

FIG. 7B describes the dependence of the specific detectivities of thetwo detectors on the power density at different gate voltages. At gatevoltages from 0 V to 60 V, the specific detectivities of thephotodetector based on TiO₂/MoS₂ are all significantly higher than thoseof the device based on MoS₂. At zero-gate voltage, the maximumdetectivity of the photodetector based on TiO₂/MoS₂ is 1.7×10¹³ Jones,which is about 3.2 times that of the device based on MoS₂ (4.0×10¹²Jones), as shown in FIG. 7D. In addition, as the gate voltage increasingfrom 0 V to 60 V, the detectivities of the two photodetectors are allreduced significantly, this is because that at greater gate voltages,the dark current is higher, thus reducing the specific detectivity.

As shown in FIG. 8A-FIG. 8B, an energy band alignment model oftriple-layer MoS₂ and TiO₂ is employed to illustrate the mechanism ofphotoresponse. There are hole-traps at the edge of MoS₂ valence bands,which may be charge traps introduced by structural defects of MoS₂ suchas vacancies or dislocations as well as MoS₂/SiO₂ interfaces. As shownin FIG. 8A, for the photodetector based on MoS₂, under visible lightillumination, MoS₂ absorbs photon energy and excites electron-holepairs. The photo-generated holes are then captured by traps, whichprolong the life of photo-generated electrons in the conduction band,generate a local gate voltage of a certain magnitude (corresponding tothe negative shift of the threshold voltage), and induce more electronsin the channel, thereby enhancing the photoresponse. The defect levelscaused by the oxygen vacancies in TiO₂ are located at 1.2 eV below thebottom of the conduction band, which can act as electron traps. As shownin FIG. 8B, TiO₂ and MoS₂ form type I energy band alignment, which helpsthe transport of electrons from TiO₂ to MoS₂. The high photoresponse ofthe photodetector based on TiO₂/MoS₂ can be explained in two ways. Onthe one hand, visible light can excite electrons captured in trap statesof TiO₂ to make them transit to the conduction band, and then transferinto the MoS₂ conduction band, so that the Fermi level of MoS₂ canincrease slightly, thereby increasing the electron concentration in theMoS₂, further increasing the current in the channel, and improving themodulation of the gate voltage on the photocurrent. On the other hand,TiO₂/MoS₂ interfaces will introduce a great amount of hole traps intoMoS₂, thus increasing the density of trap states, which may generate ahigher local gate voltage under light illumination, thereby attractingmore electrons into the conduction band and improving the optical gain.

In conclusion, the present disclosure employs a micromechanicalexfoliation method of tearing and sticking with adhesive tapes (but notlimited to this method) and a van der Waals integration method bysite-specific transferring of electrodes to construct a back-gatedfield-effect transistor based on MoS₂, then an e-beam evaporationtechnology is used to deposit the metal Ti at a thickness of 2 nm on thesurface of the MoS₂ channel, and then a photodetector based on TiO₂/MoS₂heterojunction can be obtained after natural oxidation. Through themethod of the present disclosure, a relatively perfect Au/MoS₂ interfacecan be obtained, and this method can avoid the damage to the MoS₂lattice or the introduction of chemical impurities; at the same time,the strong wettability of Ti increases the contact area between TiO₂ andMoS₂, and the oxygen vacancies generated from the incomplete oxidationof Ti improve the responses of TiO₂ to the visible light. Under visiblelight illumination, the photodetector based on TiO₂/MoS₂ exhibits a highphotoresponsivity of 1099 A/W and a high specific detectivity of1.67×10¹³ Jones, which are increased by 1.7 times and 3.2 timesrespectively compared to those of the device based on MoS₂.

The present disclosure proposes a simple method for preparing thevisible light detector with high-photoresponse based on TiO₂/MoS₂heterojunction, which can be extended in commercial applications at alow cost and in a high efficiency. However, it should be noted that, theproduction rate of MoS₂ in the micromechanical exfoliation process istoo low, so it is difficult to prepare in large scale. Therefore, inindustrial applications, chemical vapor deposition or physical vapordeposition can be used to prepare MoS₂ thin films with large area. Thisstudy not only provides a good foundation for the application oftransition metal dichalcogenides in photoelectric detection, but alsoprovides a new idea for the development of novel high performancephotodetectors.

Although several examples of the present disclosure have been givenherein, it should be understood by the technical persons in the art thatvariations can be made to the examples herein without deviating from thespirit of the present disclosure. The above examples are only exemplary,rather than being considered as the limitation on the protection scopeof the present disclosure.

What is claimed is:
 1. A visible light detector with high-photoresponsebased on a TiO₂/MoS₂ heterojunction, wherein the detector is based on aback-gated field-effect transistor based on MoS₂, the detectorcomprising a MoS₂ channel, a TiO₂ modification layer, a SiO₂ dielectriclayer, Au source/drain electrodes and a Si gate electrode, the TiO₂modification layer being modified on the surface of the MoS₂ channel,the TiO₂ modification layer being obtained from e-beam evaporation of acertain thickness of metallic Ti film on the surface of the MoS₂ channeland natural oxidation of the metallic Ti film, the MoS₂ including a fewlayers of MoS₂ flakes with a high crystallinity.
 2. The visible lightdetector with high-photoresponse based on TiO₂/MoS₂ heterojunctionaccording to claim 1, wherein the MoS₂ flakes are in a hexagonal phasewith a single-crystal structure, the few layers numbering 3-5 layers,and the overall thickness is 2-2.5 nm.
 3. The visible light detectorwith high-photoresponse based on TiO₂/MoS₂ heterojunction according toclaim 1, wherein the TiO₂ modification layer is a naturally oxidizedTiO₂ layer having a thickness of 1-2 nm.
 4. The visible light detectorwith high-photoresponse based on TiO₂/MoS₂ heterojunction according toclaim 3, wherein the TiO₂ layer is in a crystalline state or anamorphous state, and when the TiO₂ layer is in the crystalline state, ithas single-crystal sheets.
 5. The visible light detector withhigh-photoresponse based on TiO₂/MoS₂ heterojunction according to claim1, wherein at a zero-gate voltage and under illumination of awhite-light LED, the detector can reach a high photoresponsivity of 1099A/W and a high specific detectivity of 1.67×10¹³ Jones.
 6. A method ofpreparing the visible light detector with high-photoresponse based on aTiO₂/MoS₂ heterojunction, wherein the method comprises the followingsteps: S1. preparing MoS₂ flakes, and transferring the MoS₂ flakes ontoa SiO₂/Si wafer; S2. constructing a transistor based on MoS₂,site-specific transferring gold electrodes onto the MoS₂ flakes obtainedin step S1, getting source/drain electrodes of the detector, ahighly-doped Si substrate being a gate electrode; S3. e-beam evaporationof Ti, depositing a certain thickness of metallic Ti film on a channelsurface of the transistor based on MoS₂ constructed in step S2, gettinga device based on the Ti/MoS₂ heterojunction; and S4. natural oxidation,exposing the device based on the Ti/MoS₂ heterojunction prepared in stepS3 in air for oxidation, obtaining the TiO₂/MoS₂ heterojunction for avisible-light detector.
 7. The method of preparing the visible lightdetector with high-photoresponse based on the TiO₂/MoS₂ heterojunctionaccording to claim 6, wherein, in step S1, the MoS₂ flakes are preparedby micromechanical exfoliation, and the MoS₂ flakes are heated afterbeing transferred onto the SiO₂/Si wafer.
 8. The method of preparing thevisible light detector with high-photoresponse based on the TiO₂/MoS₂heterojunction according to claim 6, wherein, in step S2, the electrodethickness of the transistor based on MoS₂ is 50 nm, and after thetransistor based on MoS₂ is constructed, it is annealed at 200° C. in anatmosphere of Ar/H₂ at 10 Pa for 1 hr.
 9. The method of preparing thevisible light detector with high-photoresponse based on the TiO₂/MoS₂heterojunction according to claim 6, wherein in step S3, the thicknessof the e-beam-evaporated Ti film is 2 nm, and the deposition rate is 0.2Å/s.